The present invention relates to the field of solid state lighting, and in particular to a plurality of LED strings coupled to a common power source in parallel and comprising a non-dissipative current balancer.
Light emitting diodes (LEDs) and in particular high intensity and medium intensity LED strings are rapidly coming into wide use for lighting applications due their high efficiency, long life, mechanical compactness and robustness, and low voltage operation, without limitation. LEDs with an overall high luminance are useful in a number of applications including backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a matrix display, as well as for general lighting applications. Due to the limited power capacity of a single LED device, in many applications multiple LEDs are connected in series to form an LED string. The constituent LEDs of an LED string thus share a common current.
In a large LCD matrix display, and in large solid state lighting applications, such as street lighting, typically the LEDs are supplied in a plurality of strings of serially connected LEDs, at least in part so that in the event of failure of one string at least some light is still output.
LEDs exhibit similar electrical characteristics to diodes, i.e. they only conduct current when the forward voltage across the device reaches its conduction threshold, denoted Vf. When the forward voltage rises above Vf the current flowing through the device increases sharply. As a result, a constant current source is preferred for driving LEDs, typically implemented as a switching type DC to DC converter in current control mode.
LEDs providing high luminance exhibit a range of forward voltage drops, denoted Vf, and their luminance is primarily a function of current. For example, one manufacturer of LEDs suitable for use with a portable computer, such as a notebook computer, indicates that Vf for a particular high luminance white LED ranges from 2.95 volts to 3.65 volts at 20 mA and an LED junction temperature of 25° C., thus exhibiting a variance in Vf of greater than ±10%. Furthermore, the luminance of the LEDs vary as a function of junction temperature and age, typically exhibiting a reduced luminance as a function of current with increasing temperature and increasing age.
In order to provide a balanced overall luminance, it is important to control the current of the various LED strings to be approximately equal despite the disparate electrical characteristics of the various strings. In one embodiment a power source is supplied for each LED string, and the voltage of the power source is controlled in a closed loop to ensure that the voltage output of the power source is consonant with the voltage drop of the LED string, however the requirement for a power source for each LED string is quite costly.
In another embodiment, as described in U.S. Patent Application Publication US 2007/0195025 to Korcharz et al, entitled “Voltage Controlled Backlight Driver” and published Aug. 23, 2007, the entire contents of which is incorporated herein by reference, this is accomplished by a controlled dissipative element placed in series with each of the LED strings. In another embodiment, binning is required, in which LEDs are sorted, or binned, based on their electrical and optical characteristics. Thus, in order to operate a plurality of like colored LED strings from a single power source, at a common current, either binning of the LEDs to be within a predetermined range of Vf is required, or a power regulation device, such as the dissipative element of the aforementioned patent application, must be supplied to drop the voltage difference between the strings caused by the differing Vf values so as to produce an equal current through each of the LED strings. Either of these solutions adds to cost and/or wasted energy.
U.S. Pat. No. 7,242,147 issued Jul. 10, 2007 to Jin, entitled “Current Sharing Scheme for Multiple CCF Lamp Operation”, the entire contents of which is incorporated herein by reference, is addressed to a balancer, wherein each CCFL is connected to an AC power source lead via a primary transformer winding. The secondary windings are connected in a closed in-phase loop. The balancer requires an alternating current input in order to avoid DC saturation of the transformers, and is thus not suitable for use with LED strings, which operate only on DC.
What is needed is a LED driving arrangement, preferably of low cost, which further provides appropriate balancing between the LED strings without excess power dissipation.